1. Field of the Invention
The present invention relates to a surface acoustic wave device and, in particular, to a surface acoustic wave device in which an insulating film is arranged to cover electrode films constituting at least an IDT (interdigital transducer).
2. Description of the Related Art
In known RF surface acoustic wave filters, LiTaO3 substrates and LiNbO3 substrates are used as piezoelectric substrates. To improve the frequency-temperature coefficients of the piezoelectric substrates, different structures have been proposed in which electrode films constituting IDTs are formed on piezoelectric substrates, and thereafter, SiO2 films are formed on the piezoelectric substrates so as to cover the electrode films (e.g., Japanese Unexamined Patent Application Publication Nos. 2-37815 (Patent Document 1), 8-265088 (Patent Document 2), and 9-186542 (Patent Document 3), and WO96/4713 (Patent Document 4)).
The surface acoustic wave devices having SiO2 films formed therein to improve the frequency-temperature coefficients as described above have problems in that the characteristics are deteriorated since the upper surfaces of the SiO2 films have convex portions and concave portions.
On the other hand, in the case in which the upper surfaces of the SiO2 films are flattened as described in Patent Document 4, the reflection on electrode fingers or the like is reduced, so that the characteristics are improved to some degree. However, a process of flattening the upper surfaces of SiO2 films is required, or a step of forming an SiO2 film of which the upper surface is flat becomes necessary. Thus, the production method tends to become complicated.